The present invention relates to a service channel signal transmission system and, more particularly, to a service channel signal transmission system for a radio communication line of the heterodyne relaying type or direct relaying type which uses a plurality of radio channels.
A service channel signal is required for a heterodyne type of radio relaying line to monitor and control the relaying line. It has been extensively practiced to transmit a service channel signal together with desired data through a common radio communication channel, that is, without resorting to any radio channel for exclusive use. Each intermediate repeater station has a modem for interruption which is adapted to receive various commands and control signals necessary for its own station, while sending out reports and monitored data. The modem is connected to the radio channel for the transmission of the service channel signal.
Hereinafter will be described, by way of example, the service channel signal transmission system for heterodyne relaying or repeating type 4-phase PSK modulation digital radio communication lines, one for regular use and the other for spare use, which has a transmission capacity of 34.368 Mbit/s and is used at a 7 GHz radio frequency band.
A main signal is transmitted in the following manner. A 34.368 Mbit/s signal is converted at a terminal station into a 4-phase PSK signal having a modulation rate of about 17 MHz by a 4-phase PSK modulator. This 4-phase PSK signal modulates a 70 MHz intermediate frequency to provide an intermediate 70 MHz frequency whose frequency spectrum width is about 17 MHz. The intermediate frequency is modulated by a transmitter into the radio frequency of a carrier of the radio communication line and then transmitted therefrom. The radio frequency may be 7428 MHz for regular use and 7477 MHz for spare use, for example. A repeater station has a receiver which amplifies the received wave and demodulates it into the 70 MHz intermediate frequency. This 70 MHz intermediate frequency having a component demodulated by the main signal is delivered to a transmitter, modulated by the transmitter to a radio frequency different from the previously mentioned carrier frequency, and then transmitted to another repeater station or a terminal station. This time, the radio frequency may be 7589 MHz for regular use and 7638 MHz for spare use, for example.
A service channel signal, on the other hand, is inserted by further frequency-modulating the radio carrier with the component modulated by the main signal by means of the service channel signal. At a terminal, the service channel signal is separated from the main signal by a 4-phase demodulator; at a repeater, it is demodulated by a service channel signal demodulator and by partly branching the intermediate frequency which contains the component modulated by the main and service channel signals. The service channel signal transmission system described above has been widely used for service channel signal transmission on various kinds of digital radio communication lines, regardless of the applied radio frequency, transmission capacity, repeating system, or even modulation system which is not limited to the described 4-phase PSK but may be 2-phase PSK, 8-phase PSK, 16 QAM, 32 QAM, 64 QAM, FSK or .DELTA.M, for example. Sending a service channel signal utilizing a radio line for communication has also won popularity in the art of analog radio communication.
In the prior art service channel signal transmission system discussed above, as a fault occurs in the radio channel for service signal transmission, a channel switchover control signal (generally referred to as "AL tone") exchanged between terminal stations is monitored to switch over the connection of the demodulator and the radio channel to the spare radio channel. However, this is undesirable from the economic viewpoint because each terminal station has to be furnished with a channel switchover control unit and each repeater station with an instrument for monitoring the channel switchover control signal.
Another known implement for coping with faults consists in delivering a service channel signal constantly to two radio channels in parallel and connecting the demodulator of each intermediate repeater constantly to one of the radio channels. When an alarm signal appears indicating that the received signal at a receiver in the radio channel is abnormal, the demodulator is switched over to the other radio channel. Such a system suffers from the drawback that the repeaters following the repeater which has generated the alarm signal are disenabled to receive the service channel signal. Generally, the signal-to-noise ratio exceeds a predetermined allowable value in response to a drop of the received input level (referred to as "threshold level" hereinafter). As a certain repeater receives a signal which is lower than the threshold level, it generates an alarm signal and cuts off its received signal output circuit. A transmitter associated with this radio channel includes a carrier reinsertion circuit for inserting a non-modulated carrier so that no alarm signal is caused to appear in the next repeater station and onward. Therefore, the demodulator in the next repeater station cannot be switched over.